Answer:
The water plant
Step-by-step explanation:
According to this question, Mason performs an experiment where he places a water plant in a test tube of water and turns it upside down in a rack by a sunny window. However, the next day, he observed that there were bubbles in the water as well as in the top of the tube.
These bubbles were actually OXYGEN GAS produced as a result of a metabolic process carried out by the water plant called PHOTOSYNTHESIS. In photosynthesis, which is the process whereby plants synthesize their food in the presence of sunlight, OXYGEN gas is given off as a waste product. The oxygen gas is what was observed by Mason as bubbles. Hence, the bubbles were produced by the WATER PLANT.